De-duplicate README content

[matt-graham]

Currently there is some duplication of content across the top-level README.md file, the index.rst file used as the documentation site index page and the .pip_readme.rst file used in the packages PyPI index page, which runs the risk of one or more getting of date / out of sync with each other.

It's possible to include Markdown files in a Sphinx site using either extensions like https://github.com/omnilib/sphinx-mdinclude and https://myst-parser.readthedocs.io/en/v0.17.2/sphinx/intro.html so we could potentially include the top-level README.md in the Sphinx docs and also use this as a direct replacement for the .pip_readme.rst file by just pointing to this file in pyproject.toml once #173 is merged.